A. Technical Field
The present invention relates to a process for the purification of exhaust gases. Specifically, the invention relates to a process for the purification of exhaust gases which process enables a long-time stable removal of harmful substances (e.g. dioxins) that are contained in exhaust gases from incinerators.
B. Background Art
Many of already existing incinerators are types for the purification of their exhaust gases with an electric dust collector or cyclone to thereby dedust the exhaust gases, and then discharging the exhaust the exhaust gases into the air. However, the merely dedusted exhaust gases contain harmful substances such as dioxins (in the present invention, these harmful substances are referred to as dioxins) and therefore involve socially serious problems.
As to the removal of the dioxins, effective processes are developed, and the problems of the dioxins can considerably be solved by newly planting incinerators which utilizes these new technologies.
However, it is economically difficult to immediately dismantle the already existing incinerators and then newly plant the incinerators which utilizes the new technologies. Therefore, it is desired to enable to solve the problems of the dioxins by utilization of already existing incineration facilities without construction for much modification.
A. Object of the Invention
An object of the present invention is to provide a process for the purification of exhaust gases which process enables to effectively inhibit the discharge of the dioxins by low-cost modification of already existing incineration facilities which are formed from an incinerator and an electric dust collector or cyclone.
B. Disclosure of the Invention
The present inventors have found from their studies the following:
(1) The dioxins can effectively be removed by placing a catalyst downstream from the electric dust collector or cyclone.
(2) The dust collection efficiency of the electric dust collector or cyclone is so bad that the dust removal is insufficient. The catalyst can decompose gaseous dioxins, but cannot remove either dioxins contained in dust or particulate dioxins. The dioxins are usually measured in total of dusty, particulate, and gaseous dioxins. As a result, therefore, a sufficiently high dioxins-removing ratio cannot be achieved.
(3) Setting a bag filter in place of the electric dust collector or cyclone might be though of, but its displacement construction costs much. The bag filter exhibits so high a dust collection efficiency as to be able to remove most of the dusty and particulate dioxins. However, the operational temperature of the bag filter is so low that the amount of the catalyst needs to be increased for sufficiently decomposing the dioxins with the catalyst. As a result, there are economical disadvantages. In addition, the low operational temperature involves the inclusion of sulfur oxide in the exhaust gases and therefore causes problems of catalyst deterioration. Incidentally, a method in which exhaust gases purified by the bag filter is heated and then introduced into the catalyst might be thought of, but the heating results in an increase of costs.
(4) The dioxins can effectively be removed from exhaust gases by setting a ceramic filter downstream from the electric dust collector or cyclone in order to enhance the dedusting efficiency, thereby sufficiently removing dust from exhaust gases, and then introducing the exhaust gases into the catalyst. Furthermore, the durability of the catalyst can also thereby be enhanced.
(5) If exhaust gases are purified with a ceramic filter which supports a catalyst, then the resultant effects are the same as those obtained in the above case where the exhaust gases are dedusted with a ceramic filter and then brought into contact with the catalyst.
The present invention has been completed on the basis of the above findings.
That is to say, a process according to the present invention for the purification of exhaust gases comprises the steps of:
(A) dedusting incinerator exhaust gases with an electric dust collector or cyclone and then
(B) bringing the exhaust gases into contact with a catalyst and
(C) treating the exhaust gases with a ceramic filter.
Specifically, for example, the above process according to the present invention for the purification of exhaust gases can preferably be executed by: a first process in which step (B) is carried out after step (C); or a second process in which step (B) is carried out at the same time as step (C); or a third process in which step (C) is carried out after step (B).
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.